Recently, a thin-film multilayer substrate for aiming a high-density mount and high-speed signal processing of semiconductors has been watched with keen interest and, in general, a substrate formed by laminating a copper foil as an electrically conductive layer and a polyimide resin layer as a supporting layer is used. On such a substrate, a device is mounted, and since the device generates heat due to its characteristics, it has been desired to use a substrate material having a high heat-radiating effect such that heat does not accumulate in the inside of the system having incorporated therein the device.
As such a substrate, various circuit substrates having heat spreader have been proposed and for preparing these substrates, a so-called successive laminating method of using a sheet-form metal substrate having a high heat-radiating effect, coating a liquid resin on the substrate followed by drying, and thereafter, forming thereon a conductor layer, a press laminating method of press-working a copper-applied laminate layer plate and a metal plate having a large heat-radiating effect, etc., have been proposed.
However, the former method has a problem that since the step number for working is large, the cost becomes high and the latter method has a problem that since the accuracy for position matching is rough, it is difficult to obtain fine lines. Since these problems are fundamental problems in the practical use, the production method for the substrate with high reliability, high density, and low cost has not yet been actually realized.
On the other hand, from the points of obtaining a high storage capacity and high speed of a hard disk, in place of the conventional thin-film magnetic head which is MIG or a magnetic induction type, a so-called MR-thin-form composite type head formed by integrating a magnetic resistance-type MR element and a thin film has been watched with a keen interest. While in a conventional magnetic head, the reading and the writing of magnetic signals are both made by one head, since in the MR head, the reading and the writing are divisionally made by one head, the number of terminal becomes twice (if necessary, an earth terminal is further added) and it becomes necessary to fine the wire connecting the head to the disk body. However, when the wire is fined, the wire becomes liable to be corroded and also there occur the problems that it becomes reluctant to conform the impedances and also it becomes difficult to mount the head.
As a method of solving such new problems, a method of directly forming circuits on a suspension on which a head is mounted as described in JP-A-48-16620 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, if the materials constituting the substrate have different heat expansion coefficients (linear expansion coefficients), the substrate warps on the generation of heat. Also, if the water absorption of the polyimide resin layer being used for the circuit-forming substrate described above is high, for example, when such a substrate is incorporated in a hard disk body as a suspension, the dimensional change with the absorption or the desorption of water to or from the polyimide resin layer becomes large, the suspension itself warps to reduce the alignment accuracy, and it is also considered that there is a possibility of changing the interval between the disk and the disk body to cause an inferior performance as a device.
Also, the circuit-forming substrate or the circuit substrate is prepared by a sheet-form step of forming a polyimide resin layer and a conductor layer in order on a long stainless steel foil, and since the number of the production steps is large and the steps are complicated, the cost for producing the substrate is high.
Furthermore, if a three-layer substrate composed of a copper-applied laminated layer plate (for example, a two layer substrate composed of a polyimide resin layer and a copper foil) and a long stainless steel foil can be available, for pattern working of the polyimide resin layer, it is necessary to apply dry etching such as plasma etching, laser abrasion, etc. As a result, there is a possibility that the dry etching gives damages to the stainless steel foil and other wiring circuits and the cost becomes high with the reduction of the throughput. On the other hand, it may be considered to work the polyimide resin layer by wet etching but in this case, a toxic chemical such as hydrazine, etc., must be used as the processing liquid and hence the foregoing method is undesirable in the environmental sanitation.
Also, a circuit-forming substrate is not always used in a flat board state but as the case may be, it sometime happens that after making a circuit substrate by forming circuits on the circuit-forming substrate, the circuit substrate is processed by partially bending the circuit substrate at about 90 degree. When such a processing is applied, a stress is applied to the bent portion and thus there is a possibility that the polyimide resin layer constituting the substrate is cracked to cause an inferior insulation.